This invention is directed to an apparatus for converting between the TM.sub.01 and TE.sub.11 circular waveguide modes over a wide operating bandwidth.
The TM.sub.01 mode is the mode often generated by high power microwave sources with axial electron beams, but its electric field distribution makes it difficult to use. In order to best use such sources, it is necessary to convert to the TE.sub.11 mode.
One type of mode convertor, known as the "serpentine" mode convertor, is described in "Design of a single-period 8.6 GHz TM.sub.01 -TM.sub.11 serpentine mode convertor and TM.sub.01 and TE.sub.11 bends in a moderately overmoded circular waveguide," G. H. Luo et al., Digest of the SPIE International Conference on Infrared and Millimeter Waves, Dec. 1990. This type of device converts the TM.sub.01 mode to the TE.sub.11 mode by a periodic series of bends in the circular waveguide. The serpentine design is very narrow band and relatively long (approximately 10 waveguide diameters). It also has no provision for collecting an electron beam if required.
A second type of mode convertor is a "tapered radial-fin" design as described in "Development and High-Power Testing of a High Efficiency, Broadband TM.sub.01 -TE.sub.11 Mode Convertor," C. David Cremer et al., Proceedings of the Fifth National Conference on High Power Microwave Technology, 10-15 Jun. 1990, pp 284-287, and U.S. Pat. No. 4,999,591. This approach has wide bandwidth but at the expense of extensive extraneous mode contamination in both the input and output ports. This may be acceptable in some applications, however not in all, and particularly not where a uniform, high density radiation region is desired. This design can provide an extraneous-mode-free output over a smaller 12% bandwidth but the input port would always have extraneous modes. These extraneous modes act as energy absorbers creating a very irregular passband characteristic and reducing the conversion efficiency. The claimed efficiency averages about 70%.
It is therefore an object of the present invention to provide a well-matched, wideband, extraneous-mode-free mode conversion between the TM.sub.01 and the TE.sub.11 circular waveguide modes.